The present invention relates to fluid controllers of the type used to control the flow of fluid from a source of pressurized fluid to a fluid pressure operated device, and particularly, to such controllers which are used to control the flow of pressurized fluid to a vehicle steering cylinder.
A typical fluid controller of the type to which the present invention relates includes a housing, which defines various fluid ports, and further includes valving operable in response to an input, such as the rotation of the vehicle steering wheel. This typical controller also includes an arrangement, such as a fluid meter, for imparting follow-up movement to the valving in response to the flow of fluid through the fluid meter. The flow of fluid through the controller valving, the fluid meter, and to the steering cylinder, is generally proportional to the rate of rotation of the steering wheel.
Fluid controllers of this general type receive pressurized fluid at the inlet port, control the flow of pressurized fluid through a main variable flow control orifice, then to the fluid meter, and from the fluid meter pressurized fluid flows through another variable orifice, and then to the expanding chamber of the steering cylinder. Fluid being expelled from the contracting chamber of the steering cylinder flows through the controller valving and then to the system reservoir.
The flow path described above is typically referred to as the "main fluid path", and in theory, all parts of this flow path, upstream of the steering cylinder, contain pressurized fluid during any particular operation of the fluid controller. However, there are various operating conditions which occur periodically, wherein not all parts of the main fluid path contain pressurized fluid, as is normally desired.
Occasionally, the steering cylinder is subjected to an "overrunning" load, wherein an external force is applied to the cylinder, tending to bias it in the same direction as it is being displaced by the pressurized fluid in the main fluid path. When the steering cylinder is subjected to an overrunning load, it is common for cavitation to occur, wherein a void or "bubble" occurs in the main fluid path, upstream of the steering cylinder. Such a cavitation void may not result in an immediate problem, but typically, such a void migrates into the fluid meter, and at the time of the next steering operation, the presence of the void in the fluid meter can, in an extreme condition, result in loss of steering control. It is important to note that, between the time that the cavitation void occurs, and the time of the next steering operation, the controller valving has typically returned to the neutral position.
Another operating condition which occurs periodically is operation in the manual steering mode. This typically occurs when there is an engine or pump failure, such that there is no pressurized fluid available at the inlet port of the fluid controller. In that case, the vehicle operator rotates the steering wheel with sufficient manual input force to cause the fluid meter to operate as a hand pump, pumping fluid through the remainder of the main fluid path to the steering cylinder.
In either of the operating conditions described above, it is necessary to supply fluid to the main fluid path, and this has typically been done by means of anti-cavitation and manual steering check valves disposed in the housing of the fluid controller. Typically, these check valves are disposed to permit fluid to flow from the return port of the controller into the inlet port, (manual steering checks) or into the work port (anti-cavitation checks), whenever the fluid pressure at the inlet port, or the work port, is below that of the return port. The performance of the prior art arrangement described above has been generally satisfactory, although the machining of the necessary bores and passages to provide for the manual steering check valves represents a substantial increase in the complexity and expense of the controller housing. In the case of the anti-cavitation check valves, these have typically been housed in a port block valve (bolted to the port face of the controller, which also represents a substantial increase in the expense and overall size of the controller.
One functional disadvantage of the typical prior art arrangement of the manual steering check valves being in the controller housing is that the fluid flowing past the check valve has typically had to flow through the series arrangement of variable flow control orifices in the main fluid path. These orifices are fully open during a normal steering operation and are closed when the valve is in neutral. If a bubble is trapped in the fluid meter, when the fluid controller returns to neutral, steering may be lost upon steering wheel input.